It is well known that digital systems tend to generate radio frequency noise at harmonics of the clock frequencies of logic within the system, this is a result of switching transients throughout the system. In systems with fixed clock frequencies, this noise can be significant enough at particular frequencies to cause problems with other electronic devices, including radios and similar high-gain systems; in some systems this noise may result in sufficient electromagnetic radiation to cause issues with regulatory agencies.
A technique that has been used to reduce noise at particular peak frequencies is to modulate the clock frequency, “spreading” the clock frequency into a band. Spreading the clock frequency also spreads spectrum of the radiated harmonics from switching transients in the system, with result that intensity at nominal harmonic frequencies is reduced at the expense of increased noise at nearby frequencies. Clock generators that provide a frequency spreading function are referred to herein as spread-spectrum clock generators.
Some prior clock-frequency-spreading systems use a phase-locked loop to dither phase, and hence frequency, by shifting phase between the voltage-controlled oscillator (VCO) of the phase locked loop and the phase detector of the phase locked loop; U.S. Pat. No. 8,593,228 FIG. 8 discloses a system of this type. Accumulated phase shift at the phase detector causes a modulation on the VCO control voltage, modulating VCO output frequency periodically and slowly.
Some integrated circuits are mixed-signal integrated circuits that rely on sampling of analog signals at precise times, or on transitioning signals into analog circuitry at precise times. Clock jitter, such as would result if a VCO of a PLL clock generator driving an analog clock were to vary in frequency, could result in an undesirable noise-equivalent in sampled-data circuitry; for example clock jitter in a delta-sigma digital-to-analog converter (DAC) could cause noise on the DAC output. In typical mixed-signal integrated circuits, digital switching noise is a much stronger contributor to electromagnetic interference than is analog switching noise.